The present invention relates to an improved press for producing boards such as chipboards, fiberboards, and the like. My recently issued U.S. Pat. No. 4,004,873 discloses the general type of press which the present invention also relates to. To the extent necessary for an understanding of the present invention, the subject matter of this patent is incorporated herein by reference thereto.
The above-noted patent relates to a continuously operating press, especially a prepress, for a mat for manufacturing board, such as chipboard, fiberboard, or the like, with two endless-plate conveyors or belts, consisting of plates articulated together, said conveyors being mounted one above the other and traveling about axes located horizontally and parallel to one another, the sections of said conveyor facing one another being drivable in the same direction by pressure rollers. Each of the endless-plate belts are surrounded by an endless steel belt or pressing band which is pressed against the material being pressed by way of the belt plates. In each of the chambers surrounded by the endless pressing band (viewed in the direction of motion of the adjacent sections of the endless-plate belt) at least one high pressure roller is disposed beyond the endless-plate belts, the diameter of said high pressure rollers being greater than the diameter of each pressure roller. The high pressure rollers which produce a high-pressure zone and are surrounded by the respective two endless pressing bands are mounted in the same vertical plane. At least one of the high pressure rollers is drivable, and at least one of these high pressure rollers is movable in the direction of the other with an adjustable pressure force, and the pressure between the high pressure rollers is much higher than the pressure between the pressure rollers acting on the plate belt (low-pressure zone).
Although presses of the type referred to above have operated satisfactorily, especially when used as prepresses, they suffer from the disadvantage that the sliding elements disposed between the outlet of the endless-plate belts and the high pressure rollers forming the high-pressure zone, said sliding elements being designed to prevent the prepressed mat from springing off, are subject to relatively rapid wear. This can be eliminated by providing the surfaces of the sliding elements which face the endless steel belt (pressing band) with a coating which reduces the frictional forces which develop, for example, a coating of Teflon. Such a solution has certain advantages, but is not an optimum solution for all cases.
Therefore, an object of the invention is to improve the above-mentioned presses in such manner that there is practically no further wear of the parts which are designed to prevent the mat from springing off at least beyond the endless-plate conveyors. Moreover, the present invention is designed to prevent springing off of the material beyond the high pressure rollers that form the high-pressure zone. These and other objects are achieved according to the invention by surrounding each of the high pressure rollers forming the high-pressure zone with a high pressure endless belt disposed between the roller and the endless steel belt (pressing band). In particularly preferred embodiments, the high pressure belt is formed of polyamide material with said high pressure belt being guided over a reversing roller, said reversing roller being designed as a pressure means for the high pressure belt, being provided against and next to the endless steel belt (pressing band) surrounding the corresponding endless-plate belt. Particularly preferred embodiments also include the provision that the endless polyamide belts are of a thickness such that springing off of the mat pressed in the low-pressure zone is prevented. This is accomplished by converting the previous sliding friction into rolling friction, with much less resistance. Tests have shown that endless polyamide belts should have a thickness of about 4 mm. Although such an endless polyamide belt 4 mm thick is relatively stiff, long-term tests have shown that this belt is not damaged, although it is deformed around radii of curvature between 225 mm and 750 mm. Thus, for example, the rollers acting as the pressure means against the polyamide belt on the endless steel belt have a diameter of 450 mm, the high pressure rollers forming the high-pressure zone have a diameter of 1500 mm, and the reversing rollers provided with control means have a diameter of 660 mm; in other words the radius of curvature of the endless polyamide belt is constantly undergoing change.
Furthermore, it is provided in preferred embodiments that a second reversing roller is present for each endless polyamide (high pressure) belt, said second reversing roller being provided with control means to prevent the endless polyamide belt from shifting sidewise.
According to other preferred embodiments, three reversing rollers are provided for each high pressure endless polyamide belt, wherein the third reversing roller as viewed from the direction of movement of the corresponding endless polyamide belt is disposed beyond the high pressure roller forming the high-pressure zone and the spacing of the rotational axes of this third reversing roller must be set such that the space between the outer endless belts (pressing bands), which preferably consists of steel or plastic, is precisely the same size in the vicinity of the third reversing roller as in the vicinity of the high pressure rollers that form the high-pressure zone. This arrangements provides that the mat, compressed by the high pressure rollers forming the high-pressure zone, is able to spring back only on a delayed basis, which is particularly important in the manufacture of thick boards to avoid cracks. If the continuously operating press is stopped for some reason, the part of the mat which is influenced by the high-pressure rollers forming the high-pressure zone is subject to practically linear influence and this differential thickness of the mat would lead to crack formation when the press was started again. These cracks are advantageously avoided by providing the third reversing roller beyond the rollers forming the high-pressure zone.
It is also contemplated according to other preferred embodiments of the invention, to go one step further to improve the long-term resistance of the endless polyamide belts (high pressure belts) by providing more than three rollers for each endless polyamide belt and ensuring that the length of each endless polyamide belt is greater than the circumferential length of each of the endless-plate belts and smaller than the length of the corresponding outer endless belt. This reduces the number of changes caused by the variations in the radius of curvature of the reversing rollers and also results in a more uniform mat or a more uniform board path, because the polyamide belts which surround the endless-plate belt distribute over their outer surfaces the pressure exerted by the endless-plate conveyors and the pressure rollers. Moreover, the mat is prevented from springing back in the area between the endless-plate conveyors and the following rollers by this arrangement.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, several embodiments in accordance with the present invention.